Use of interferometric apparatus for generation of optical gratings is well known in the art. Such apparatus usually includes an interferometer which divides a collimated coherent beam of light into two component beams that traverse different optical paths. The component beams are then made to intersect by reflecting one or both of the beams. The intersecting component beams form an interference pattern. The pattern has a high or low intensity depending upon whether the light waves of the two component beams are in-phase (constructive interference) or out-of-phase (destructive interference). The regions of constructive and destructive interference are planes oriented parallel to the bisector of the angle between the two intersecting beams and perpendicular to the plane defined by the beams. The spacing between the light and dark planes is determined by intersection angle between the waves and their wavelengths. The grating-like interference pattern is recorded in a photoresponsive material such as film, photoresist, or other. Linear and reciprocal photoresponsive materials do not require a beam having a high intensity per unit area so that the grating size can be readily increased by increasing the cross-sectional area of the beam with the use of a telescope. Once the beam is spread by the telescope, a collimator lens or mirror is used to align the rays in the beam parallel to each other. A large collimator lens or mirror is either very costly or impossible to make.
When it is desired to generate a grating-like pattern on a nonlinear and nonreciprocal photoresponsive material such as polymethyl metharcylate, or when it is desired to initiate a photochromic process with a two-photon absorption, the intensity of the beam required to efficiently obtain the grating-like pattern has to be so high that only extremely small patterns can be generated with presently available laser sources. Because of limited power of presently available laser sources, a grating-like pattern cannot be formed on a large area unless extremely long exposure times are used.
Therefore, it is an object to generate a large grating-like pattern.
Another object is to provide apparatus for generating a grating-like pattern where the size of the pattern is independent of the light source power.
Yet another object is to provide an inexpensive apparatus for generating a large grating-like pattern.
These and other objects are realized in an illustrative embodiment of the invention in which apparatus for piecewise generation of a grating-like pattern on a photoresponsive material with a coherent beam of light includes an interferometer wherein the coherent beam is split into two component beams which are made to intersect in a photoresponsive material having a planar surface positioned normal to the bisector of the angle between the two intersecting component beams. The intersecting component beams form a parallel plane interference pattern on a section of the photoresponsive material. An arrangement, which translates the coherent beam of light and the component beams parallel to themselves, generates the interference pattern on a different section of the photoresistive material.
A feature of the invention is the arrangement for translating the coherent beam and the component beams parallel to themselves.
Another feature is the formation of the interference pattern in a different section of the photoresistive material.
Another feature is the formation of the interference pattern in overlapping sections of the photoresponsive material.
Another feature of the invention is the adjustment of the difference between the optical path lengths of the two component beams.
Yet another feature of the invention is the adjustment of the angle between two intersecting component beams by rotation of a single element about a fixed axis.